Related to 1.05.06 Draft 3 + MQTT-JSON Plugfest. 6.2.3.2.2 DataTypeSchemaHeader says:
"The DataTypeSchemaHeader of the DataSetMetaData shall be populated with the necessary
information. This includes all namespaces and DataTypes that are potentially contained in the
associated DataSetMessages. DataTypes defined in the OPC UA namespace
(NamespaceIndex 0) that are not built-in types shall be included in the DataSetMetaData."

It is possible to interpret this to mean only the "leaf types" used in the metadata should be added to the header. However, e.g. StructureDefinition contains NodeId for the parent type, thus the parent type should be added as well (and recursively meaning the whole hierarchy).

I would like that a clarification would be added such as "including their supertypes recursively i.e. the header should contain the minimum full inheritance hierarchy for all related types; builtintypes excluded".

While the decoding of a Structure is possible already with just the "leaf information" since they are self-contained, the representation of that could potentially use the full inheritance information. For example, the Prosys OPC UA SDK for Java preferably represents Structures via the "most code-generated type in the inheritance hierarchy" when possible.